1. Field of the Invention
The present invention relates to modulation of radiation in optical fibers. More specifically, the present invention relates to production of temporally smooth broadband pulses of optical radiation by cross phase modulation of light in an optical fiber.
2. Description of Related Art
The word "laser" is an acronym for "Light Amplification by Stimulated Emission of Radiation". Laser radiation has application in a wide variety of disciplines, such as communications, medicine, the military, research, and any other field where directed electromagnetic radiation is an advantage. The light produced from a laser has many known applications, and it is reasonable to expect that many applications of the laser have yet to be discovered. A typical laser comprises three basic elements: a resonating cavity, a gain medium, and a means to pump the gain medium.
The resonating cavity of a laser may comprise two or more opposed mirrors that reflect electromagnetic radiation (such as light). One of the mirrors typically has less than 100% reflectivity so that a portion of the light will be transmitted and the remainder will be reflected. The output of the laser passes through this mirror, which is sometimes termed the "output coupler". For example, an output coupler may have 90% reflectivity, which means that 10% of the incident optical energy will be transmitted, and the remainder (90%) will be reflected.
The gain medium of a laser may comprise any of a variety of materials: solid materials such as Nd:YAG or Er:YAG, gases such as KrF, CO.sub.2 or Ar.sup.+, and liquids such as dye. The gain material absorbs energy from the pump, storing that energy in the form of higher energy states in the molecular or sub-atomic level.
Due to the smallness of the optical wavelengths, a simple laser cavity such as that described above can support oscillation at many different wavelengths. A laser cavity resonator may oscillate simultaneously at several wavelengths, and in several "temporal modes", or oscillation may alternate between one or more of the modes competing for the gain of the laser. The spectral content of such a laser is irregular; the bandwidth may be large but the intensity is principally divided among the several and sometimes thousands of modes of oscillations. The output pulse intensity from such a multi-mode, broadband laser is typically characterized by a highly modulated, rapidly fluctuating shape. For many applications, an output at a single wavelength and mode is desirable, and much research is currently being devoted to design lasers whose output is exactly single wavelength. The lasers that come closest to a "single wavelength" oscillate in a single temporal and spatial mode and have a very narrow bandwidth around that single wavelength.
Very high power pulsed lasers may comprise a number of lasers or power amplifiers connected together. A "seed", or "master" oscillator generates a laser output which is provided to one or more power amplifiers. This configuration may be termed "MOPA" (Master Oscillator-Power Amplifier). The seed oscillator may provide a coherent beam (constant phase) of collimated light, or it may provide an incoherent beam (random phase) to the following power amplifiers.
If a coherent beam is desired, a master oscillator will produce light with a single wavelength and a single spatial mode which can be effectively amplified by the lasers or the power amplifiers. However, the master oscillator must produce a beam that is extremely coherent, both temporally and spatially, and single wavelength to a very high degree. If other wavelengths or modes are present in the output from the master oscillator, interaction between the various temporal and spatial modes by diffraction processes will lead to localized areas of intense radiation. These localized areas may lead to damage to components carrying that radiation if the intensity at that localized area is above the damage threshold of the component, such as amplifier material or mirrors or lenses. Therefore, high power lasers have been conventionally operated at an output intensity three or four times less than the damage threshold, to allow for the localized areas where the intensity may be greater. When a single-mode master oscillator is used, the laser can operate much closer to its damage threshold without damaging optical components. Use of a single-mode master oscillator with a smooth temporal pulse shape is common practice with high power laser systems such as the current solid state laser system at Lawrence Livermore National Laboratory called "NOVA".
If a high degree of coherence of the output beam is not required, incoherent light within the laser system may be advantageous. Since totally incoherent light has a random spatial and temporal phase, there are no diffraction or interference patterns formed. As a result, there would be no areas where the localized intensity may vary greatly from the average intensity, and thus the laser could operate close to its damage threshold.
However, using totally incoherent light in a high power laser amplifying chain is difficult because the amplified light will rapidly diverge as it propagates through the laser chain, which results in loss of power, loss of ability to focus, and other problems. In spite of these problems, significant applications exist for high power incoherent light. One such application is inertial confinement fusion, where total incoherence of laser light on target may provide significantly improved efficiency in coupling the beam to the target. Such an application requires that the output laser beam or beams be both spatially and temporally incoherent. In practice, temporal incoherence alone can be obtained by amplifying broadband laser light with a laser amplifier chain.
As discussed above, in a laser system each material through which the laser passes has a damage threshold which describes the peak electric field amplitude or intensity of the laser pulse that can pass through the system without damage to the components. Average power output from the laser is severely limited by rapid intensity fluctuations. Therefore, to avoid damage to the components while increasing average output power, time fluctuations in intensity should be minimized. Minimizing (i.e., smoothing) the peak intensity fluctuations of a temporally incoherent pulse over time can provide a high average power to the target, because the pulse can propagate through the system at an average intensity just below the damage threshold.
Additionally, spatial incoherence of the light is required for certain target irradiation experiments related to laser-driven inertial confinement fusion (ICF) of deuterium and deuterium/tritium filled spherical target shells. It is now understood in the ICF field that laser irradiation non-uniformity on targets must be less than a 1% root-mean-squared (rms) deviation from the average intensity over the target surface. Focussed radiation from today's solid-state or gas laser systems can not achieve this degree of intensity uniformity on target. In addition, local laser radiation "hot-spots" on the target can cause many undesirable light scattering instabilities in the under-dense coronal plasma surrounding the target sphere. These plasma instabilities cause severe scattering of the incoming laser light away from the target, causing further radiation and plasma nonuniformities, thereby preventing target compression and nuclear fusion.
Therefore, conversion of high power laser systems with coherent beams into target irradiation sources that can direct incoherent light onto target is highly desirable in the ICF field.
Various systems have been developed at major laboratories for converting a pulse of coherent light into a pulse of incoherent light. These systems require that the pulse have a certain finite bandwidth for optimal incoherence conversion. Each system has different bandwidth requirements. For example, a system developed at the University of Rochester-Smoothing by Spectral Dispersion (SSD)--requires a 2 .ANG. to 4 .ANG.bandwidth (FWHM), while a system developed at the Naval Research Labs-Induced Spatial Incoherence (ISI)--requires a 20 .ANG. to 30 .ANG. bandwidth (FWHM). The ISI system is described in an article by R. H. Lehmberg and S. P. Obenschain, "Use of Induced spatial Incoherence for Uniform Illumination of Laser Fusion Targets", Optics Communications, Vol. 46, No. 1, June 1, 1983, pp. 27-31, and in another article by Lehmberg, et al., "Theory of Induced Spatial Incoherence", J. Appl. Phys. Vol. 62, No. 7, Oct. 1, 1987, pp. 2680-2701. The SSD system is described in an article by Skupsky, et al., "Improved Laser-Beam Uniformity Using the Angular Dispersion of Frequency-Modulated Light", J. Appl. Phys. Vol. 66, No. 8, Oct. 15, 1989, pp. 3456-3462.
It would be advantageous to have a laser system that can provide a specific bandwidth reliably and conveniently while maintaining a smooth temporal pulse shape. Furthermore, it would be an advantage if a single laser system were available that could be adjusted to provide any of a number of bandwidths, such as 2 .ANG., 4 .ANG.or 20 .ANG.. For research using these and other methods, it is desirable to be able to conveniently and continuously vary the bandwidth of the pulse.
For other applications, such as laser pulse compression, it is desirable to have a pulse that has a broad bandwidth while retaining a single temporal and spatial mode. It is advantageous if the pulse has a spectral content that is approximately evenly distributed around the central wavelength. It is a further advantage if the pulse's intensity varies smoothly in time, for safe laser operation at high power.
A wide bandwidth means a wide variation from the pulse's center wavelength. A temporally smooth pulse has an intensity as a function of time that does not change abruptly. Such a temporally smooth pulse has an average intensity that is typically close to the peak intensity of the pulse.
The initial pulse to the NOVA laser amplifiers is produced in a master oscillator system. The pulse is then amplified in several amplifier stages through which it is passes. Then, the amplified pulse may be frequency doubled or tripled in nonlinear crystals before it is focussed on a target. The present invention may be included as a portion of a master oscillator system for a laser that has a master oscillator-power amplifier (MOPA) configuration.